- Name also: Wild Calla, Marsh Calla, Water Arum, Water-arum, Water-dragon
- Family: Arum Family – Araceae
- Growing form: Perennial herb. Rootstock almost horizontal, creeping, thick. Often forms wide stands.
- Height: 10–40 cm (4–15 in.). Stem soft.
- Flower: Perianth vestigial, flower regular, approx. 5 mm (0.2 in.) broad. Stamens usually 6. Gynoecium fused, single-styled. Inflorescence a dense, abundantly flowered 2–3 cm long spadix; lower part with bisexual flowers, upper part with unisexual staminate flowers. Inflorescence’s subtending leaf (spathe) white, outer surface greenish, ovate, long-tipped.
- Leaves: Alternate at base, long-stalked. Base sheath-like. Blade ovate, cordate-based, with entire margins, glabrous, shiny.
- Fruit: Red, angular, juicy, approx. 5 mm (0.2 in.) broad berry.
- Habitat: Swamps, waterside bogs that are prone to flooding, stream banks, ditches, quagmires, shores of muddy lakes.
- Flowering time: June–July.
Bog arum is the sole Finnish representative of the mainly tropical Arum family. It is a close relative of many popular Finnish house-plants such as spotted dumbcane, peace lily, hardy arums, Swiss cheese plant and anthurium, and it looks similar especially with regards to its inflorescence. Bog arum’s real flowers are very inconspicuous: small, greenish and lacking tepals. The inflorescence’s large, white spathe however makes it more attractive, at least to people, and perhaps also attracts pollinators. The flower’s fragrance is quite weak and unpleasant to humans, but it attracts an abundance of wetland flies, beetles, thysanopterans and even molluscs such as snails. The flowers develop juicy berries, which float easily to new habitats. Later on the berries disintegrate because the slime that has been secreted from the seed stalk expands as it becomes waterlogged and finally breaks the shell that is covering the seed. The sticky slime helps the seeds attach to e.g. water-birds and find new habitats. Bog arum also spreads well through parts of its rootstock that break off and even when patches of the plant are ripped up.
All parts of bog arum, including the berries, are poisonous, and the rootstock in particular contains toxic compounds, the composition of which is not yet fully understood. At least most of the poison disappears, however, when the berries are boiled or dried and it has been used in hard times to feed cattle and stretch out flour. The root is surprisingly nutritious and it is said to be the best wild emergency food. There is even mention of collecting the rootstock in the Finnish national epic poem Kalevala: when Ilmarinen steals the youngest daughter of the North, she complains of having had “to go to the bog to pick cranberry and bog arum“. Bog arum’s submerged and damp habitats are often undeniably hard to reach: along with bogbean and marsh cinquefoil it is one of Finland’s most notable plants for closing up waterways. Bog arum is a very common plant in wetlands as far as Kainuu and around Oulu, but it becomes rarer around the Arctic Circle. Individual daredevils can be found as far north as northern Lapland, however.